fix: computeTrend division by zero, add tests for trend and currency parsing, add username to settings

internal/bot/salary.go

@@ -6,6 +6,7 @@ import (
 	"math"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/go-telegram/bot/models"
 
@@ -179,6 +180,20 @@ func (h *Handler) handleSetRateMsg(ctx context.Context, msg *models.Message) {
 	h.sendText(ctx, msg.Chat.ID, fmt.Sprintf("Hourly rate set to %s%.2f", currencySymbol(st.Currency), st.HourlyRate))
 }
 
+// parseCurrencyInput validates and normalizes currency input (symbol + code).
+func parseCurrencyInput(s string) (string, string) {
+	parts := strings.Fields(s)
+	if len(parts) < 2 {
+		return "", "Please provide both symbol and code.\nUsage: /setcurrency <symbol> <code>\nExamples:\n/setcurrency $ USD\n/setcurrency T Toman\n/setcurrency IRR Rial"
+	}
+	for _, p := range parts[:2] {
+		if len(p) > 10 || strings.ContainsFunc(p, unicode.IsControl) {
+			return "", "Invalid currency. Use short printable strings like $ USD, T Toman, IRR Rial."
+		}
+	}
+	return parts[0] + " " + parts[1], ""
+}
+
 // handleSetCurrencyMsg handles /setcurrency <symbol> <code>.
 func (h *Handler) handleSetCurrencyMsg(ctx context.Context, msg *models.Message) {
 	args := strings.TrimSpace(strings.TrimPrefix(msg.Text, "/setcurrency"))
@@ -192,20 +207,11 @@ func (h *Handler) handleSetCurrencyMsg(ctx context.Context, msg *models.Message)
 		h.sendText(ctx, msg.Chat.ID, fmt.Sprintf("Current currency: %s\nUsage: /setcurrency <symbol> <code>\nExamples:\n/setcurrency $ USD\n/setcurrency T Toman\n/setcurrency IRR Rial", st.Currency))
 		return
 	}
-	parts := strings.Fields(args)
-	if len(parts) < 2 {
-		h.sendText(ctx, msg.Chat.ID, "Please provide both symbol and code.\nUsage: /setcurrency <symbol> <code>\nExamples:\n/setcurrency $ USD\n/setcurrency T Toman\n/setcurrency IRR Rial")
+	currency, errMsg := parseCurrencyInput(args)
+	if errMsg != "" {
+		h.sendText(ctx, msg.Chat.ID, errMsg)
 		return
 	}
-	if len(parts[0]) > 10 || strings.ContainsFunc(parts[0], func(r rune) bool { return r < 32 || r > 126 }) {
-		h.sendText(ctx, msg.Chat.ID, "Invalid currency symbol. Use a short printable string like $, €, £.")
-		return
-	}
-	if len(parts[1]) > 10 || strings.ContainsFunc(parts[1], func(r rune) bool { return r < 32 || r > 126 }) {
-		h.sendText(ctx, msg.Chat.ID, "Invalid currency code.")
-		return
-	}
-	currency := parts[0] + " " + parts[1]
 	user, err := h.getOrCreateUser(msg.Chat.ID)
 	if err != nil {
 		h.sendText(ctx, msg.Chat.ID, "Error loading profile")